The generation of electricity from wind power has become commonplace over the last two decades. The wind, as a renewable energy source is free and, with the advent of certain technologies such as advanced power conversion and microprocessors, wind power has become competitive with more traditional power generation methods. In its infancy, wind energy relied principally on asynchronous generators which were directly connected the utility grid system. Because the utility is fixed voltage and frequency, these generators and the turbines to which they were connected ran at essentially constant speed. The speed varied only in so far as the wind caused the generator to traverse its torque-slip curve.
Constant speed operation of wind turbines was effective for many years. However, it eventually became recognized that running a wind turbine at variable speed could enhance the ability of the turbine to capture energy while providing reduction in mechanical loads. With this recognition, came the idea of running wind turbines at variable speed in such a way so as to maintain a constant tip speed ratio on the rotor blades when the wind turbine is operating below rated power. This requires increasing the rotor speed in direct proportion to the wind speed. In using this approach an additional 4–6% energy capture was achievable with a turbine depending on certain site parameters such as mean wind speed and turbulence. However, to implement such a turbine effectively requires the introduction of a variable speed solid state converter. Examples of such converters are included in U.S. Pat. Nos. 5,083,039, 5,225,712, 5,155,375, 6,137,187, and 6,420,795, all of which are incorporated herein by reference.
The introduction of variable speed also presented additional advantages for the turbine design. For example, by appropriately controlling the torque on the turbine's generator, mechanical loads could be reduced in the wind turbine structure. Reduction in mechanical loads is particularly beneficial for reducing the size, weight, cost, and rated capacity of the wind turbine's gearbox. This allows for less material to be used in the turbine and a lower capital coast can be achieved.
Solid state converters for providing variable speed operation are important in dealing with the utility interconnection power quality. In the early constant speed turbines, the asynchronous generator consumed reactive power from the utility grid for excitation. This reactive power caused voltage regulation problems with local utilities as significant reactive power consumption can reduce utility system voltage. At certain wind farm sites where a large number of turbines are sited, this is a significant problem and power factor correction capacitance must be installed to supply the reactive power requirements of the generators. Variable speed turbines, with their solid state power converters inserted between the generators and utility grid, are capable of supplying the asynchronous generators with their reactive power and eliminating this need from the utility system. Further, these solid state converters are capable of controlling the reactive power at the utility interconnection point and they can supply or absorb reactive power in such a way as to support other loads connected to the utility system. In this way, these turbines have become helpful to the utilities.
U.S. Pat. Nos. 5,083,039 and 5,225,712 which are incorporated herein by reference discuss the use of a power converter to control an asynchronous generator system on a turbine. These devices require an active rectifier system to decouple the magnetic field and torque components of the generator. The system is effective at providing high bandwidth torque response and accuracy. The utility inverter is connected to the active rectifier via a DC link which decouples the voltage and current operation on the generator from the utility. The utility side converter is pulse width modulated (PWM) to provide a high quality, low harmonic distortion current wave form back into the utility. The turbine-converter system is designed to meet the high quality power standards such as IEEE 519 with the wind turbine AC connection serving as the point-of-common-coupling (PCC). In a large wind farm with a large number of turbines, the belief is that connecting a large number of turbines which individually meet utility power quality requirements will result in the wind farm itself meeting the utility power quality requirements with the PCC being the utility transmission system. The extension of the windturbine to wind farm power quality may or may not be factual as there are other items in the wind farm which are capable of adversely affecting the wind farm power quality. An example would be turbine transformers which absorb reactive power and which can distort the current wave form as the core of the transformer moves into or near saturation. Additionally, supplying or absorbing reactive power within the wind farm has the penalty of increasing the load on all of the current carrying conductors within the confines of the wind farm.
U.S. Pat. Nos. 6,137,187 and 6,420,795 incorporated herein by reference are other examples of the variable speed wind turbine systems which utilize an asynchronous generator and a solid state power converter. These devices differ from that of U.S. Pat. Nos. 5,083,039 and 5,225,712 however, in that the converter supplies the wound rotor of the generator with the stator directly connected to the utility grid. In this case, the converter carries only a portion of the total power and can be downsized depending upon the slip range that the generator operates over. Since most benefits of energy capture and load mitigation can be realized with a 1.5:1 speed range, the generator can operate as low as 30% subsynchronous slip to 30% supersynchronous at the high speed. This then allows the converter to be rated at approximately 30% of the total turbine power rating. In this system, reactive power can be controlled by over or under exciting the wound rotor. This is done by separating the current in the rotor into a magnetizing and torque producing components. By under exciting the magnetizing component of the rotor circuit, the generator absorbs reactive power from the stator and utility connection. As excitation is brought up on the rotor, less and less reactive power is supplied by the utility to the stator. Finally, at the unity power factor point, the generator is fully excited from the rotor and no reactive power is supplied by the utility. If the rotor magnetic excitation is increased further, then the generator begins to supply reactive power back to the utility. Just as in U.S. Pat. Nos. 5,083,039 and 5,225,712, the power quality is measured and met at the wind turbine, not necessarily at the wind farm.
Although the art described focuses power quality on the wind turbine itself, it is customary in wind farm installations for the PCC to be defined as the point of connection between the entire wind farm and the utility transmission system. The invention described herein presents a low cost, robust integration of the wind farm collection system and the included multiple turbine converters. The invention identifies the power quality PCC as the wind farm interconnection point and not as the turbines themselves. In small installations the PCC would be the utility distribution system and in larger wind farms, greater than a few megawatts, this would be at the utility subtransmission level.